“C-V-T” type capacitive distance sensors are disclosed for the first time in a Chinese invention patent with the title of “Capacitive Distance Sensor” and the application number of 201210403271.2. In the “C-V-T” type capacitive distance sensors, measuring circuits are simplified and under the same process conditions, the technical level of such sensors is higher than the internationally advanced level in 2012. As the technology of Touch ID sensors installed in iPhone5S is proposed by Apple Inc. in 2013, the development and application of fingerprint sensor technologies are greatly pushed forward, and the consumer electronics market has huge demand and higher requirements for the fingerprint sensor technologies.
Technical schemes of the capacitive fingerprint sensors disclosed in a Chinese invention patent with the title of “Capacitive Fingerprint Sensor” and the application number of 201410004072.3 lie in that a sensing equation and a circuit model are optimized on the basis of the Chinese invention patent with the title of “Capacitive Distance Sensor” and the application number of 201210403271.2, and “C-Q-T” conversion is realized in the proposed circuit structures. However, there are several problems described below.
First, it can be observed from the sensing equation Vt′=(Vs−Vt)*(Cd+Cg)/(Ct+Cd+Cg) that variation Vt′ of voltage Vt of an integrating capacitor during each iteration is negatively correlated with Vt. On one hand, Vt′ causes gradual decrease of Vt, which further leads gradual decrease of Vt′, while smaller Vt′ is likely to be interfered by noise. Thus, a comparator flip will become either earlier or later, resulting in noise interference with output of the sensor. On the other hand, a relation between output time of the sensor and target capacitors Cg is convex nonlinear. The convex nonlinear relation may serve as a compensation measure for convex nonlinear conversion from distances to capacitance within a certain spatial scale, which has been explained in the above two Chinese invention patents with the titles of “Capacitive Distance Sensor” and “Capacitive Fingerprint Sensor”. With the development of technologies, the thickness of a dielectric layer between a sensing electrode and a target electrode in a commercial capacitive fingerprint sensor is increased from an order of magnitude of 10 um to an order of magnitude of 100 um, so that the distance-to-capacitance conversion relationship falls into an approximately linear region. Thus the convex nonlinear conversion from output of the target electrode to output of the fingerprint sensor has become a disadvantage.
Meanwhile, in a technical scheme of the Chinese invention patent with the application number of 201410004072.3, a bus parasitic capacitor is calculated into the integrating capacitor, improving the discreteness of the integrating capacitor. As a background capacitor in a sensing unit is not removed from the sensing equation, manufacturing tolerance of the background capacitor will cause inconsistence among units in a fingerprint sensor array.